DISORDER IN LAYERED TICL3 - SYMMETRY OF THE COORDINATION SITE

The structural layers of TiCl3 consist of two monoatomic layers of chlorine atoms octahedrally coordinated to intercalating Ti atoms. Mechanical (grinding) and/or chemical treatment of the regular alpha- or gamma-forms may produce the disordered delta-form, wherein shifts and rotations between neighboring layers give rise to the largest disorder compatible with close-packing of the chlorine layers, accompanied by reduction of crystallite size both along the layers and across them. A Bernoullian statistical model fully describes the statistical structure of the polycrystalline samples in terms of 4 parameters, two of them relating with the stacking disorder, the other two with crystallite size distribution. The catalytic sites for isospecific alpha-olefin polymerization lie on the side edges of the structural layers. It is shown that Ti atoms on these edges possess two isochiral free valencies, enabling them to exchange the roles of the (Ti-coordinated olefin) and of the (Ti-growing chain) bonds at each polymerization step with no change of the local chirality, in analogy with the metallocene-based isospecific catalysts.